The present invention relates generally to semiconductor manufacturing and, more particularly, to a diffusion furnace used in a diffusion process for forming an oxide film on a semiconductor wafer by thermal oxidation.
Diffusion furnaces have been used to form oxide films on semiconductor substrates. Some diffusion furnaces are configured to mix a dilution gas in an oxidizing gas such as water vapor, and thermally oxidize the mixture to form an oxide film on the semiconductor wafer. Various examples of oxide forming apparatus that employ diffusion furnaces are illustrated in FIGS. 1-3.
As shown in FIG. 1, a diffusion furnace apparatus 10 includes a torch heater 12 for heating H2 from line 14 and O2 from line 16 to a temperature that is higher than the ignition point for H2. Water vapor or steam is generated from the H2 and O2 in the external torch chamber 20. A dilution gas is added to the water vapor via a dilution gas line 24 prior to entry into the furnace tube 26 for thermal oxidation to form the oxide film on one or more semiconductor wafers inside the furnace tube 26. An exhaust 28 is provided for the gas to exit the furnace tube 26. Because the dilution gas is colder than the steam, the mixing of the colder dilution gas with the steam may cause condensation.
FIG. 2 shows a diffusion furnace apparatus 30 which heats the dilution gas prior to mixing with the steam. For convenience, the same components have the same reference characters in FIG. 2 as in FIG. 1. In FIG. 2, a heated dilution gas line 34 introduces a heated dilution gas into the steam line prior to entry into the furnace tube 26. This apparatus 30, however, requires an additional heater and quartz piece to provide the heated dilution gas.
In another diffusion furnace apparatus 40 shown in FIG. 3, the dilution gas 42 is introduced through the H2 line 14 or the O2 line 16. This approach is not desirable for forming thin oxides which require very low gas flow. The need to maintain a very low gas flow will cause the torch flame to be unstable and lead to flame out problems.
The present invention is directed to providing a dilution gas in a diffusion furnace apparatus for forming an oxide layer on a semiconductor wafer. In some embodiments, the dilution gas is mixed with steam and the mixture is thermally oxidized to form the oxide film on the semiconductor wafer. The dilution gas is preheated prior to mixing with the steam to avoid condensation problems. The dilution gas is heated by an existing heater in the external torch chamber or combustion chamber used to produce the oxidizing gas such as steam, so that no additional heater is needed. The preheated dilution gas is mixed with the steam at the outlet of the external torch chamber or combustion chamber so as not to cause any disturbance to the stable flame in the chamber. The dilution gas flow desirably is sufficiently low so that it is possible to form a very thin oxide layer with uniform thickness.
An aspect of the present invention is directed to an apparatus for supplying a diluted process gas into a diffusion furnace for forming an oxide layer on a substrate in the diffusion furnace. The apparatus comprises a torch device configured to receive one or more inlet gases supplied by one or more inlet gas lines. The torch device includes a torch heater configured to generate an oxidizing gas by heating the inlet gases in a torch chamber disposed downstream of the torch heater. A dilution gas line is configured to receive a dilution gas. The dilution gas line extends through the torch chamber to permit heating of the dilution gas by the heat in the torch device without mixing the dilution gas and the oxidizing gas in the torch chamber. A mixing region downstream of the torch chamber is configured to receive and mix the oxidizing gas and the heated dilution gas prior to entry into the diffusion furnace.
In some embodiments, the oxidizing gas comprises steam generated from O2 and H2 in the torch chamber. The dilution gas is typically Ar or N2. The torch heater is configured to produce a flame in the torch chamber to generate the oxidizing gas from the inlet gases. The dilution gas line is configured to produce a dilution gas flow of at most about 20 slm.
Another aspect of the invention is directed to an apparatus for supplying a diluted process gas into a diffusion furnace for forming an oxide layer on a substrate in the diffusion furnace. The apparatus comprises an oxidizing gas chamber configured to receive one or more inlet gases supplied by one or more inlet gas lines, and a mechanism for heating the one or more inlet gases in the oxidizing gas chamber to generate an oxidizing gas. A dilution gas line is configured to receive a dilution gas. The dilution gas line extends through the oxidizing gas chamber to permit heating of the dilution gas by the heat in the oxidizing gas chamber without mixing the dilution gas and the oxidizing gas in the oxidizing gas chamber. A mixing region downstream of the oxidizing gas chamber is configured to receive and mix the oxidizing gas and the heated dilution gas prior to entry into the diffusion furnace.
Another aspect of the present invention is directed to a method of supplying a diluted process gas into a diffusion furnace for forming an oxide layer on a substrate in the diffusion furnace. The method comprises supplying one or more inlet gases into a chamber, and heating the one or more inlet gases in the chamber to generate an oxidizing gas. A dilution gas is flowed through a dilution gas line which extends through the chamber to permit heating of the dilution gas by the heat in the chamber without mixing the dilution gas and the oxidizing gas in the chamber. The oxidizing gas and the heated dilution gas are mixed downstream of the chamber prior to entry into the diffusion furnace.
In some embodiments, heating the one or more inlet gases comprises producing a flame from the O2 and H2 to generate the steam. The dilution gas flow rate is sufficiently low so that the dilution gas is heated to a temperature which is substantially equal to a temperature of the oxidizing gas before mixing the oxidizing gas and the heated dilution gas.
Another aspect of the invention is directed to a method of supplying a diluted process gas into a diffusion furnace for forming an oxide layer on a substrate in the diffusion furnace. The method comprises supplying one or more inlet gases into a chamber, and producing a flame in the chamber to heat the one or more inlet gases in the chamber to generate an oxidizing gas. A dilution gas is flowed through a dilution gas line which extends at least partially through the chamber to a location downstream of the flame to permit heating of the dilution gas by the heat in the chamber without mixing the dilution gas and the oxidizing gas at or upstream of the flame. The oxidizing gas and the heated dilution gas are mixed downstream of the flame prior to entry into the diffusion furnace.